Three major advances have been made during the last four years of AG10770 funding: First, the Neuropathology Research Laboratory (NRL) verified that recombinant MoPrP(89-230) folded into amyloid filaments are prions because they caused a neurodegenerative disease with the neuropathological features of prion disease and because the disease could be serially transmitted. Moreover, the neuropathological phenotype indicated that the synthetic prions were a new prion strain. Second, the NRL found that neurodegeneration caused by natural prion strains proceeds in a stereotypical sequence that begins with accumulation of PrPSc in presynaptic boutons, is followed within two weeks by synaptic dysfunction, degeneration of boutons, and dendritic atrophy, and finally results in nerve cell death one to two months later. For the past two years, we focused on early-occurring synaptic degeneration because it is potentially reversible and, therefore, relevant to drug therapies for prion diseases. Third, we applied discoveries of developmental neurobiology to search for alterations in neuronal proteins reported to play pivotal roles in synaptic development. We found (i) early-occurring decreased expression of GAP43 in neurons, a gene required foraxon extension and structural organization of the presynaptic bouton, and (ii) early-occurring increased expression and activation of Notch1 in neurons. During CMS development, activation of Notch1 causes regression of both dendrites and axons, similar to events in prion diseases. These data suggest that PrPSc accumulation in neurons induces programmed dendritic atrophy ("dendritic apoptosis") and programmed presynaptic bouton dysfunction and degeneration {"synaptic apoptosis"), which are analogous to PrPSc-induced programmed nerve cell death (apoptosis). The central specific aim is to test the hypothesis that PrPSc formation and accumulation in plasma membranes cause early-occurring synaptic abnormalities by activating Notch1. However, our previous studies argue that PrPSc is likely to affect synapses via alternative pathways. Therefore, we will an alternative hypothesis that posits that synaptic dysfunction and degeneration are caused by both Notch1-dependent and Notch1-independent pathways. The generation of synthetic prion strains, which have a unique PrPSc accumulation phenotype, provide us with a new tool to test whether or not the stereotypical progression of neurodegeneration found with natural RML prions is the same for all prion strains, including synthetic prion strains. Alternatively, each prion strain may trigger neurodegeneration by different molecular and cellular pathways, which may represent another defining prion strain characteristic.